1. Field of the Invention
The present invention relates to a printer, copier, facsimile apparatus or similar electrophotographic image forming apparatus and more particularly to a toner conveying device for use in the image forming apparatus and a toner replenishing device including the same.
2. Description of the Background Art
An electrophotographic image forming apparatus includes a developing device for developing a latent image formed on an image carrier with a developer. When a two-component type developer, i.e., a toner and carrier mixture is used as a developer, a toner bottle, toner cartridge, toner tank or similar toner container is positioned in the vicinity of the developing device in order to replenish fresh toner to the developing device, as needed. A full-color image forming apparatus, extensively used today, needs four developing devices and four toner containers respectively storing yellow, magenta, cyan and black toners. In addition, it is necessary with such an image forming apparatus to make the configuration compact without reducing the amount of toner to be stored in each toner container. In this respect, arranging a particular toner container in the vicinity of each developing device not only obstructs the compact configuration of the apparatus, but also noticeably limits design freedom.
In light of the above, Japanese Patent Laid-Open Publication No. 2000-81778, for example, discloses an image forming apparatus in which a suction type of screw pump, generally referred to as a Mono pump, is used to replenish toner from a toner container to a developing device. This configuration allows the toner container to be located at any desired position as a unit independent of the developing device. However, the apparatus taught in the above document has some problems left unsolved, as will be described hereinafter.
When the screw pump replenishes toner to the developing device, frictional resistance acts between toner grains and the outer periphery of a rotor, between the toner grains and the walls of spiral grooves formed in a stator and between the toner grains themselves, obstructing the movement of the toner grains and exerting a torque when the screw pump is driven. It follows that if the shape of the toner grains is amorphous far different from a sphere, then it is likely that the toner grains are caught by the outer periphery of the rotor, the walls of the spiral grooves of the stator and each other, aggravating the drive torque of the screw pump.
Further, because the suction type of screw pump sucks the toner from the toner container by generating vacuum, it sucks small, light toner grains more easily than large, heavy toner grains. Consequently, if the size of the toner grains is distributed over a broad range, then it is likely that small toner grains are replenished before large toner grains with the result that the toner grain size of the developer noticeably varies and brings about various image defects including background contamination.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 2001-249525 and 2002-62760.